In the search for high performance materials, considerable interest has been focused upon carbon fibers. The term "carbon fibers" is used herein in its generic sense and includes graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist substantially of carbon and have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit a substantially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for carbon fiber reinforced composities include aerospace structural components, rocket motor casings, deep-submergence vessels and ablative materials for heat shields on re-entry vehicles.
In the past procedures have been proposed and are generally known in the art for converting an acrylic fibrous precursor to an amorphous carbon form or to a graphitic carbon form which retains substantially the same fibrous configuration as the starting material. The acrylic fibrous material is first thermally stabilized, and then carbonized.
The thermal stabilization of acrylic fibers generally has been accomplished in the past by heating in an oxygen-containing atmosphere at a moderate temperature for an extended period of time. U.S. Pat. Nos. 2,799,915 to Barnett et al, 2,913,802 to Barnett, and 3,285,696 to Tsunoda are representative of early patents which disclose the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form by heating in air. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) a chain scission and oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups. The stabilized acrylic fibers commonly have a bound oxygen content of at least 7 percent by weight as determined by the Unterzaucher or other suitable analysis, and commonly contain about 50 to 65 percent carbon by weight.
Heretofore, fiber coalescence sometimes has been observed following the thermal stabilization reaction particularly when the acrylic precursor contains a substantial proportion of copolymerized monovinyl units with acrylonitrile groups, and/or when the stabilization reaction is carried out at a relatively high temperature. Such fiber coalescence leads to an ultimate carbon fiber product of reduced physical properties. The coalesced fibers tend to be stiff and to possess flaws at the point of coalescence even if the fibers are separated by force.
Commonly assigned U.S. Pat. No. 3,508,874 to Rulison discloses a technique for overcoming fiber coalescence by providing powdered graphite or carbon black upon the surface of the fibrous material.
An alternate approach for forming carbonized fibers directly from acrylic fibers while coated with a refractory barrier is proposed in U.S. Pat. Nos. 3,242,000 and 3,281,261 to Lynch.
It is an object of the present invention to provide an improved process for the production of carbon filaments beginning with an acrylic multifilament precursor.
It is an object of the present invention to provide an improved process for the production of carbon filaments from an acrylic multifilament precursor wherein coalescence of adjoining filaments during the thermal stabilization portion of the process effectively is eliminated.
It is an object of the present invention to provide an improved process for the production of a carbonaceous fibrous material which particularly is suitable for use as a fibrous reinforcement in a resinous matrix.
It is another object of the present invention to provide an improved process for the production of a carbonaceous fibrous material wherein the stabilization portion thereof may be conducted on a more economical basis through the use of a lesser residence time and a more highly elevated stabilization temperature in the absence of fiber coalescence.
It is another object of the present invention to provide an improved process for the production of carbon filaments from an acrylic multifilament precursor wherein a final product exhibiting superior physical properties is formed.
It is a further object of the present invention to provide an improved process for the production of carbon filaments from an acrylic multifilament precursor which satisfactorily may be carried out without fiber coalescence while employing a polymeric precursor containing a substantial quantity of copolymerized monovinyl units and/or a relatively high thermal stabilization temperature.
These and other objects, as well as the scope, nature, and utilization of the invention, will be apparent from the following detailed description and appended claims.